A flexible printed circuit is composed of an array of flexible electrical conductors mounted to a flexible electrically insulating support structure or ribbon. Such flexible printed circuits are used to electrically interconnect various components and subassemblies of electronic equipment such as computers, digital printers, video terminals, etc. Because they are flexible, these circuits can be routed along sinuous courses to effect such interconnections.
In one application with which we are primarily concerned here, such flexible printed circuits are used to releasably interconnect different printed circuit boards within a piece of electronic equipment. Accordingly, connectors are provided at the opposite ends of the flexible printed circuit. Each connector usually consists of stamped contacts captured in a plastic housing, each contact being connected to a conductor in the printed circuit. That connector is then coupled to a mating connector mounted to the printed circuit board and whose contacts connect with various printed circuit paths on that board. When the connectors are coupled together, their contacts touch or interfit, thereby establishing electrical connections between the end of the flexible printed circuit and the PC board.
Needless to say, the termination of flexible printed circuits in this fashion is a time-consuming and expensive proposition. The connectors have to be mounted to the opposite ends of the flexible circuit and connections have to be made between the various circuit conductors and the connector contacts by soldering, wirewrapping, or the like. The same thing has to be done with the mating connectors on the PC boards. Further, as a result of their construction, the prior connectors are relatively large and bulky. Therefore, they limit the number of printed circuit boards that can be packed in a given volume inside the electronic equipment. In other words, PC boards are normally arranged parallel to one another in rows or stacks and the prior connectors of this general type, because of their size, limit the minimum distance between adjacent boards.
Another problem with prior circuit assemblies of this type stems from the fact that the coupling of the flexible circuit connector to the mating connector on the PC board requires that a coupling force be applied which sometimes stresses the board to the extent that other circuit paths on the board are interrupted or damaged. Also, some prior connection assemblies do not provide adequate strain relief for the flexible circuit and some do not maintain reliable electrical connections between the circuit and the PC board when the assembly is subjected to vibration and shock.
There do exist prior connectors for flexible circuits and interconnect cables which do not require a mating connector on the PC board. One such device is disclosed in U.S. Pat. No. 4,025,142. In that arrangement, a connector is mounted to the end of the flexible printed circuit and the circuit conductors are electrically connected to the connector terminals. The connector is designed to be positioned flush against a PC board so that its terminals make contact with exposed pads on the board that terminate the board conductors. The connector is held against the board by a clamp which engages over the connector and clips to openings in the PC board.
That arrangement still has the disadvantage of requiring connections between each flexible circuit conductor and the connector. Also, that prior connector is rather bulky so that it limits the minimum distance between adjacent PC boards in a board stack. Still further, it is a relatively complicated and costly item to manufacture because it is composed of stamped metal terminals encased in a molded plastic housing.
We are also aware of an assembly for connecting a flexible circuit to a PC board wherein the flexible circuit conductors are coupled directly to the PC board termination pads thereby eliminating the need for the connector to include contacts or terminals. Such an assembly is disclosed in U.S. Pat. No. 4,054,348. Basically, the connector is a clamp member which clamps the flexible circuit contacts to the electrical pads or contacts of a PC board. The end of the flexible circuit is inserted through a slot in the connector so that the circuit contacts are exposed at the underside of the connector. Then, a pair of feet at the opposite ends of the connector are inserted through appropriate openings in the PC board near the edge thereof and the connector is pressed flush against the PC board so that the circuit and board contacts touch. A clip at the edge of the connector engages over the edge of the PC board, thereby locking the unit in place.
While that arrangement avoids the problems inherent in assemblies requiring mating connectors, it still has some disadvantages which limit its wider use and application. For example, the connector has to be installed on edge. Therefore, it limits the minimum spacing between adjacent PC boards in a stack. Also, it is difficult to insert and manipulate the connector to its locked position in a confined space. Further, the PC board conductors must be terminated adjacent an edge of the board. Moreover, when making the connection to the PC board, it is necessary to press the connector tightly against the board. This may result in bending of the board with possible damage to the board.
Also, that connector does not account for variations in the thicknesses of the materials comprising the two circuits so that its fit may not always be correct. Resultantly, it may impart undue bending stresses to the board or result in a poor connection along one or another of the conductive paths between the two circuits.